The present invention involves an innovative electrode design generally for use in instruments that measure the electrophoretic mobility of macromolecules in solution wherein the charged particles within solution are subject to an applied electric field, and their resulting motion is measured. Although the present disclosure will refer to macromolecules throughout much of its description, measurements using the inventive apparatus disclosed herein may include, more generally, all classes of small particles including emulsions, viruses, nanoparticles, liposomes, proteins, macro-ions, and any other solution constituents whose size may lie between about a half and a few thousand nanometers. Thus whenever a term such as “molecule,” “macromolecule,” “particle,” or “macro-ion” is used, it should be understood to include all of the aforementioned solution-borne objects to be subject to some form of optical measurement.
Electrophoretic mobility is the directly measurable and most widely used quantity to characterize the charge of molecules, or other particles in solution. Once measured, the electrophoretic mobility can be used in turn to determine the effective charge, Ze, carried by such molecules as well as their so-called zeta potential. The interface between the group of ions tightly bound to the particle and those of the surrounding solution that do not move with the particle defines the hydrodynamic shear plane. The zeta potential represents the electrostatic potential existing at this shear plane. It is an objective of the present invention to improve the reliability of measurements of electrophoretic mobility, effective charge, and zeta potential of molecules and particles in solution contained within a measurement cell, as well as to improve the durability of the instruments and their components.
Several techniques have been developed and are available for measuring mobilities including light scattering methods such as heterodyne DLS including both laser Doppler electrophoresis, LDE, and phase analysis light scattering, PALS. These techniques involve measuring light scattered from moving particles, whereby such scattered light carries information relating to such motion and from which the associated electrophoretic mobility of the particles may be determined.
Instruments that measure electrophoretic mobility must, by necessity, apply an electrical field, generally between two electrodes, in a fluid sample to induce electrophoresis. The resulting motion is generally probed optically to determine the resulting sample velocity. This compromises a first principles measurement of mobility, which is well established as an important parameter for predicting the stability of colloidal suspensions. In recent years electrophoretic mobility is finding new use in determining the stability of molecular solutions. Over the years, many electrode designs have been used. An objective of the present invention is to provide an inexpensive electrode that applies a uniform field and is mechanically and chemically durable.